This invention relates to the field of measuring constituent components of gas mixtures, and more particularly, to measuring the concentration of nitrogen in a gas mixture.
U.S. patent application Ser. No. 09/550,431, entitled xe2x80x9cSystem and Method to Determine Thermophysical Properties of a Multi-Component Gasxe2x80x9d and U.S. patent application Ser. No. 10/237,492 entitled xe2x80x9cA System and Method to Determine Thermophysical Properties of a Multi-Component Gas at Arbitrary Temperature and Pressurexe2x80x9d describe systems and methods for measuring heating value and energy flow rates of natural gas. An advantage of these methods is that they do not require a full composition assay. The methods correlate the speed of sound in the gas to the heating value of the gas, which depends on the gas composition, flow temperature, and pressure. The calculations for determining the heating value require known values for the diluent concentrations in the gas, such as carbon dioxide and nitrogen. However, there is no need for other constituent values to be known.
The concentration of nitrogen in a natural gas mixture is difficult to measure directly. Nitrogen has low infrared absorption characteristics, which makes infrared sensing methods difficult. Also, it is chemically inert, which makes electrochemical sensing methods difficult.
Yet, the amount of nitrogen in a gas mixture does affect heating value. Experimentation has indicated that a plus or minus shift of 0.075 mole % in nitrogen concentration will produce a plus or minus shift of 1.0 BTU/SCF in standard volumetric heating value. Thus, to make use of the above-described methods for relating speed of sound and diluent measurements to heating values, some means for measuring nitrogen content is important.
This is but one example of a need for measuring nitrogen content in a natural gasxe2x80x94there may be other applications in which measurement of nitrogen content may be useful. In the absence of satisfactory sensors for direct measurement, indirect measurement methods have been considered.